Attentuator for borehole acoustic waves

Abstract

An attenuator for use in attenuating tube waves in a borehole includes a body formed from a permeable material having a rigid matrix which can be saturated with fluid. In its simplest form, the attenuator is one or more cylindrical bodies as part of a borehole acoustic logging system. Alternatively, the shape, particularly at the end regions, can be modified to reduce the amount of reflection of tube waves in the borehole. Another attenuator includes a body for placement in a borehole formed from an attenuating material, the shape of the body being selected such that the change in impedance experienced by acoustic waves in the borehole is gradual rather than abrupt. One particularly preferred shape for the body comprises one in which the diameter is greatest towards the center of the body, for example the shape formed by two cones placed base to base, or a cylinder having conically tapered ends. A number of bodies can be used where appropriate.

We claim: 
     
       1. Apparatus for use in a borehole comprising an acoustic wave attenuator which comprises a body which is formed from a permeable material having a substantially rigid matrix having insignificant compressibility under typical borehole pressures and is positionable in the borehole so as to be in pressure communication with fluid in the borehole. 
     
     
       2. Apparatus as claimed in claim 1, wherein the porous material is natural rock. 
     
     
       3. Apparatus as claimed in claim 1, wherein the porous material is synthetic rock. 
     
     
       4. Apparatus as claimed in claim 1, wherein the porous material is a sintered particulate material. 
     
     
       5. Apparatus as claimed in claim 1, wherein the body is enclosed by a flexible sheath. 
     
     
       6. Apparatus as claimed in claim 5, wherein the permeable material is saturated with a liquid. 
     
     
       7. Apparatus as claimed in claim 1, wherein the body is elongate. 
     
     
       8. Apparatus as claimed in claim 1, wherein the body has a substantially smaller diameter than the borehole. 
     
     
       9. Apparatus as claimed in claim 1, wherein the body is surrounded by an apertured protective sleeve. 
     
     
       10. Borehole logging apparatus comprising: a) a source of acoustic signals;   b) an acoustic attenuator comprising a body formed from a permeable material having a rigid matrix having insignificant compressibility under typical borehole pressures; and   c) a receiver for acoustic signals.   
     
     
       11. Borehole logging apparatus as claimed in claim 10, wherein the attenuator attenuates tube waves which travel from the source along the borehole to the receiver. 
     
     
       12. Borehole logging apparatus as claimed in claim 10, wherein the attenuator has a smaller diameter than the borehole. 
     
     
       13. Borehole logging apparatus as claimed in claim 10, wherein the source is a monopole source. 
     
     
       14. Borehole logging apparatus comprising: a) a source of acoustic signals;   b) a receiver for acoustic signals spaced from the source; and   c) a plurality of acoustic attenuators, each comprising a body formed from a permeable material having a substantially rigid matrix having insignificant compressibility under typical borehole pressures, disposed between the source and the receiver.   
     
     
       15. A borehole acoustic attenuator, comprising an elongate body including an attenuative permeable material having insignificant compressibility under typical borehole pressures, the body having an acoustic impedance which increases towards a central portion of the body. 
     
     
       16. An attenuator as claimed in claim 15, wherein the increase in acoustic impedance is provided by a body of increasing in diameter towards the central portion thereof. 
     
     
       17. An attenuator as claimed in claim 15, wherein the shape of the body comprises two cones in a base-to-base relationship. 
     
     
       18. An attenuator as claimed in claim 15, wherein the body comprises a hollow shell filled with an attenuating medium. 
     
     
       19. An attenuator as claimed in claim 18, wherein the shell is rigid and perforated, a flexible sheath being provided over the surface of the shell to allow pressure communication with the borehole. 
     
     
       20. An attenuator as claimed in claim 18, wherein the attenuating medium comprises compressed metal fibers saturated with a viscous fluid.